The long-term objective of this research is to further characterize the adult rat ventricular cardiac muscle cell maintained in long term culture and to determine if it regains the ability to divide. Specific aims are to determine whether these cells have characteristics which are similar to the in vivo adult heart myocyte, revert back to a more neonatal cell type, are a combination of both of these cell types or have characteristics which are unique to the adult cultured myocyte. The main aim is to determine how different they are compared to the in vivo ventricular cardiac muscle cell. Parameters to be investigated include their pattern of metabolism, myosin synthesis and isoform type, Ca++ - activated myosin ATPase activity, and activity and isoenzyme pattern of creatine kinase. To assess whether these cells regain the ability to divide they will be grown on the stage of an inverted optics microscope so that individual cells can be observed over a period of days. Their behavior will be recorded using a time-lapse video recorder. Further studies will be concerned with determining how DNA replication and cell division are regulated in the ventricular myocyte using as a probe a recently isolated human thymidine kinase gene. Establishment and characterization of adult cardiac myocytes in long-term culture will provide a valuable system to study the molecular biology, biochemistry, physiology and pharmacology of the adult heart cell. If we can understand the mechanisms which control the differentiation and proliferation of the adult heart muscle cell, then it may be possible to design procedures which can be used to initiate or expedite repair or regeneration of the adult myocardium following injury such as that caused by a myocardial infarction.